[refactor] merge duplicates: combine, reject

Reject by counting expected size of merge. Combine uses a merge-with-combine fn from the underlying collection code.
Passes an initial set of tests. There’s some code we have written for completeness (MapType merge with combiner operator) which doesn’t seem to get called. Need to investigate/test further.

exist-core/src/main/java/org/exist/xquery/functions/map/AbstractMapType.java

@@ -241,4 +241,4 @@ public Sequence eval(final Sequence[] args, final Sequence contextSequence) thro
             return map.get((AtomicValue) args[0].itemAt(0));
         }
     }
-}
+}

exist-core/src/main/java/org/exist/xquery/functions/map/MapType.java

@@ -247,6 +247,43 @@ public AbstractMapType merge(final Iterable<AbstractMapType> others, final Binar
         return new MapType(context, newMap.forked(), prevType);
     }
 
+    @Override
+    public AbstractMapType merge(final Iterable<AbstractMapType> others, final BinaryOperator<Sequence> mergeFn) {
+
+        // create a transient map
+        IMap<AtomicValue, Sequence> newMap = map.linear();
+
+        int prevType = keyType;
+        for (final AbstractMapType other: others) {
+            if (other instanceof MapType) {
+                // MapType - optimise merge
+                final MapType otherMap = (MapType) other;
+                newMap = newMap.merge(otherMap.map, mergeFn);
+
+                if (prevType != otherMap.keyType) {
+                    prevType = MIXED_KEY_TYPES;
+                }
+            } else {
+                // non MapType
+                for (final IEntry<AtomicValue, Sequence> entry : other) {
+                    final AtomicValue key = entry.key();
+                    final Optional<Sequence> headEntry = newMap.get(key);
+                    if (headEntry.isPresent()) {
+                        newMap = newMap.put(key, mergeFn.apply(headEntry.get(), entry.value()));
+                    } else {
+                        newMap = newMap.put(key, entry.value());
+                    }
+                    if (prevType != key.getType()) {
+                        prevType = MIXED_KEY_TYPES;
+                    }
+                }
+            }
+        }
+
+        // return an immutable map
+        return new MapType(context, newMap.forked(), prevType);
+    }
+
     public void add(final AtomicValue key, final Sequence value) {
         setKeyType(key.getType());
         map = map.put(key, value);

exist-core/src/main/java/org/exist/xquery/functions/map/SingleKeyMapType.java

@@ -87,6 +87,12 @@ public AbstractMapType merge(final Iterable<AbstractMapType> others, final Binar
         }
     }
 
+    @Override
+    public AbstractMapType merge(final Iterable<AbstractMapType> others, final BinaryOperator<Sequence> mergeFn) {
+        final MapType map = new MapType(context, collator, key, value);
+        return map.merge(others, mergeFn);
+    }
+
     @Override
     public AbstractMapType put(final AtomicValue key, final Sequence value) {
         final IMap<AtomicValue, Sequence> map = newLinearMap(collator);